On a smooth horizontal surface at a distance $s$ from a wall lies a puck of mass $m$ at rest. A second identical puck, moving perpendicularly to the wall with speed $u$, collides with it (see top view in the figure). It is known that collisions of the pucks with the wall are elastic, and during a head-on collision between the pucks themselves, a fraction $\alpha$ ($0 < \alpha < 1$) of their total kinetic energy in the center-of-mass reference frame is dissipated.

1 Sketch a graph of the distance $l$ between the first puck and the wall as a function of time $t$, measured from the moment of the first collision between the pucks. Mark the characteristic points on it.

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